1. Field of the Invention
The present invention relates to a magnetoresistive sensor device for detecting a change in a magnetic field induced by a moving body. For example, it relates to a magnetoresistive sensor device for detecting the number of revolutions and angle of rotation of rotating objects. Specifically, the present invention relates to a magnetoresistive sensor device for detecting a change in the magnetic field induced by the moving body having a monolithic structure in which a sensing portion can be formed at any desired position above a signal processing circuit (integrated circuit IC) and can be located at the optimum position of the sensing portion for effectively detecting change in the magnetic field.
2. Description of the Related Art
In a magnetoresistive sensor device for detecting a change in a magnetic field induced by a moving body (a rotating object) such as a rotary sensor, the sensing portion is located at a position where a change in a magnetic field in the magnetic circuit comprising the moving body can be most effectively detected. The optimum position of the sensing portion in the magnetoresistive sensor device is related to 3-dimensional positional relation ships such as the structure of the moving body and magnetoresistive sensor device (the existence of a magnet, the positional relation of the magnet and the sensing portion, etc.) or the magneto-sensitive direction of the sensor element, as well as the desired form of output signal. A brief example is shown herein. FIG. 12 is a diagram for describing an example of the structure of a conventional rotary sensor. FIG. 13 is a structural diagram of a Wheatstone bridge circuit using in a rotary sensor. In FIG. 12, a magnetoresistive sensor device 23 is disposed opposite a rotating object 21 made from a magnetic body. The device 23 comprises a sensing portion 11 and a magnet 24. The sensing portion 11 is disposed in sensor element regions 12a and 12b. In FIG. 13, the Wheatstone bridge circuit comprises sensor elements 13, 14, 15 and 16. The sensor elements 13 and 16 are disposed in the sensor element region 12a, while the sensor elements 14 and 15 are disposed in the sensor element region 12b. A terminal 17 connecting to the sensor elements 13 and 15 is a power source terminal. A terminal 18 grounding the sensor elements 14 and 16 is a GND terminal. A terminal 19 connecting to the sensor elements 13 and 14 and a terminal 20 connecting to the sensor elements 15 and 16 are terminals indicating the medial point potentials between the terminals 17 and 18 respectively, and they are respectively connected to a differential amplification circuit. In FIG. 12, the rotating object 21 has an irregular form. It is composed of tooth parts (teeth) 22a and non-tooth parts (slots) 22b. The device 23 can detect a change in a magnetic field induced by the rotation of the teeth 22a and slots 22b on the rotating object 21. The difference between the magnetic field generated in the sensor element regions 12a and 12b is an output as a potential difference between medial point potential terminals 19 and 20 in the Wheatstone bridge circuit. FIG. 14 is a diagram for describing the relationship between the position of the sensing portions and the rotating object and the generated magnetic field. The optimum position of the sensing portion is shown, for example, in FIG. 14A. When the sensor element region 12a is disposed at a position corresponding to the tooth 22a of a rotating object 21, while the sensor element region 12b is disposed at a position corresponding to the slot 22b of a rotating object 21, difference in magnetic fields between the sensor element regions 12a and 12b is largest. In comparison, in the case of FIGS. 14B and 14C, the difference in magnetic fields between the sensor element regions 12a and 12b is smaller. In other words, in the example the optimum distance between the sensor element regions 12a and 12b is determined by the distance between two teeth 22a on the rotating object.
When there is a wide variety of moving body specifications (input signal specifications), in order to locate the optimum position of the sensing portion, so-called hybrid structures are often used in which, for example, the sensing portion and IC are separately fabricated to thereby provide positional flexibility to the positioning of the sensing portion. In a hybrid structure, as the sensing portion and the IC are separately fabricated, it is not necessary to consider matching in the fabrication process of the film-like magnetoresistive sensor elements often used for the magnetoresistive sensor device and the IC.
On the other hand, in recent years, higher precision sensing has been sought in magnetoresistive sensor devices as well and along with a higher sensitivity in the magnetoresistive sensor element for the purpose of improving a signal-to-noise ratio (S/N ratio), integration of the sensing portion and IC, so-called monolithic structures have been receiving attention as measures for noise reduction. Monolithic structures also have the advantages in reliability.
As noted heretofore, although hybrid structures have conventionally been used often in the structure of the magnetoresistive sensor devices, examination and practical use of monolithic structures have begun in response to recent demands for higher precision sensing. Indeed, the monolithic structures of the highly sensitive magnetoresistive sensor device have been reported on by, for example, Fukami et al (Integrated GMR Sensors for Automobiles, T IEE Japan, Vol. 120-E, No. 5, 2000). In the report, a high sensitivity giant magnetoresistive (GMR) sensor element is used as a magnetoresistive (MR) sensor element for the purpose of achieving higher precision. Furthermore, they proposed a monolithic structure having a GMR sensor element and IC in combination for solving the problems of SIN ratio in the hybrid structure. In making the move to a monolithic structure, matching of the magnetic film fabrication process and the IC fabrication process was studied. As a result, they have obtained excellent matching properties even for GMR sensor elements sensitive to the underlying conditions. The monolithic structures with the IC and the sensing portions have been achieved with exclusive regions for the sensing portions disposed within the IC. However, this structure fixes the position of the sensing portion. As a result, the specifications for detectable moving bodies are limited, thus making it difficult to respond to diverse input signal specifications.
There is also great demand for higher precision in magnetoresistive sensor devices, further accompanied with a need to respond to diversification of the detecting magnetic field (input signal specifications). For these two requirements, in the structure of magnetoresistive sensor devices, monolithic structures will be selected for increased sensitivity and hybrid structures will be selected for positional flexibility of the sensing portion. However, compatibility is difficult to achieve as these two structures are mutually incompatible. Highly sensitive elements are required for higher precision sensing. For example, these include magnetoresistive (MR) sensor elements such as giant magnetoresistive (GMR) sensor elements. Since this element exhibits sensitivity to the substrate, it is necessary to provide an exclusive region for the sensing portion within the IC as mentioned above. This fixes the position of the sensing portion and thus there is no flexibility in locating the sensing portion for a variety of input signal specifications.
A typical example of this problem occurs in vehicle-mounted revolution sensors. Vehicle-mounted revolution sensors have a rotating object facing a magnetoresistive sensor element and detect any changes in a magnetic field induced by the rotating object. Although the sensing signal is used for engine control, transmission control and the like, higher precision is being required in recent years, for example, because of more stringent exhaust gas regulations. Vehicle-mounted sensors also require to guarantees of operation under severe environments and sensor elements have realized high precision and high reliability by making then monolithic. On the other hand, with regard to rotating bodies, there exist a wide variety of specifications corresponding to each automobile manufacturer, each model or each control purpose. With conventional monolithic structures, a new IC had to be made even if same signal processing is possible, because they do not have flexible positioning of the sensing portion for different input signal specifications. The separate fabrication of ICs became a great stumbling block in manufacturing products having high efficiencies and quality, and cost reductions.
FIG. 15 is a sectional view showing an example of a conventional monolithic magnetoresistive sensor device. IC 5 is constituted by forming a circuit 1, an interlayer insulating layer 2, a first wiring 3 and an IC protective film 4 in that order on a substrate 100. A Sensing portion 11 is formed on the interlayer insulating layer 2, within the exclusive region disposed above the IC 5. In the FIG., 8 is a MR sensor element film and 9 is a protective film for the magnetoresistive sensor device.
The magnetoresistive sensor device having such monolithic structure is produced by step of first removing the protective film 4 on the exclusive region for the sensing portion 11 above the IC 5 and forming the sensing portion 11 on the interlayer insulating layer 2. The electric connection between the sensing portion 11 and the IC 5 is produced by removing the protective film 4 and then by forming the sensing portion 11 on the first wiring 3 formed on a part area within an exclusive region.